Recently, there has been an increasing interest in an environment-friendly power generation method due to an increase in power needs caused by a rapid industrial development, and concerns about an environmental pollution grown out by global warming and about an exhaustion of fossil energy.
The environment-friendly power generation method includes a fuel cell power generation method, a solar photovoltaic power generation method, a wind power generation method, and the like. In particular, the fuel cell power generation method is a representative example of the environment-friendly power generation method in that conditions for generating the fuel cell power are not much restricted as compared to the solar photovoltaic power and the wind power. Further, the fuel cell itself is environment-friendly, and little noise is made at the time of generation of the fuel cell power. An output power of the fuel cell is converted to be suitable for a load and supplied to the load or an electric power system line. Such a process necessarily requires a power conversion system (PCS).
FIG. 1 is a block diagram showing a configuration of a conventional fuel cell system.
As shown in FIG. 1, the conventional fuel cell system is made up of a fuel cell 100, a power conversion apparatus 110, and a load 120. The power conversion apparatus 110 includes a DC/DC converter 111 (hereinafter, referred to as “converter”), and a DC/AC inverter 115 (hereinafter, referred to as “inverter”).
As described above, the power conversion apparatus 110 receives a power from the fuel cell 100, suitably converts the power, and supplies the converted power to the load 120.
In this process, the DC/DC converter 111 boosts and outputs a DC voltage generated from the fuel cell 100.
Then, the DC/AC inverter 115 converts the DC voltage outputted from the DC/DC converter 111 into an AC voltage and applies the AC voltage to each load.
In order to supply the output power of the fuel cell having a low voltage•high current output characteristics to a general electric power system, a boost-type DC/DC converter and a single- or three-phase DC/AC inverter are necessarily required. However, the single-phase inverter and the three-phase inverter inevitably generate low frequency ripple components having a frequency of about 120 Hz two times higher than an output fundamental frequency and a frequency of about 360 Hz six times higher, respectively, due to a rectification effect caused by the configuration of the inverter, and the low frequency ripple components are reflected and returned to the fuel cell.
FIGS. 2A to 2C are graphs showing voltage and current values measured between the fuel cell 110 and the converter 111, between the converter 111 and the inverter 115, and after passing through the inverter 115, respectively.
As shown in FIGS. 2A to 2C, it can be seen that before passing through the converter, a voltage of the fuel cell is a low voltage but it is boosted into a high voltage through the converter and converted into an AC voltage through the inverter.
As shown in FIG. 2C, the output voltage and current are converted to have AC components through the inverter, so that the power calculated by multiplying a voltage and a current has a frequency component two times higher than a frequency of the voltage and the current. Generally, a DC link voltage is controlled to a constant value, so that a DC link current momentarily has the same value by the principle of the conservation of energy, thereby inevitably containing the AC components. The power is the same in the above-described three sections and resultantly has a form of an alternating current like the current as shown in FIGS. 2A and 2B.
That is, it can be seen from the FIG. 2A that as for the conventional power conversion apparatus 110, an input current of the converter is not constant and has an AC component (such a current will be referred to as “low frequency ripple current” hereinafter) and such a low frequency ripple current has a bad influence upon the fuel cell connected to a front end of the converter.
When the current is not constant in this manner, the fuel cell is seriously affected unlike other general apparatuses. The ripple current reduces a life span of the fuel cell and deteriorates a performance of the fuel cell. Further, as the ripple current is increased, the life span of the fuel cell is greatly reduced and the performance of the fuel cell is greatly deteriorated, and sometimes, over current may cause a malfunction, such as an emergency stop of the fuel cell system.
The present disclosure is conceived to solve the above-described problems and provides a power conversion system capable of eliminating a low frequency ripple current generated by an inverter and having a very quick response characteristics to a rapid change in a load without any overshoot or undershoot, and a control method thereof.
In accordance with an aspect of the present disclosure, there is provided a power conversion system (PCS) that converts an output of a DC power supply received from an external power supply. The power conversion system includes: a converter that converts a voltage of the input power supply; an inverter that converts an output voltage of the converter into an AC voltage; and a converter control module that outputs a PWM signal for controlling a switch of the converter. After receiving a power instruction value, the converter control module may generate the PWM signal for controlling the converter based on the power instruction value.
Further, the power conversion system may further include a first sensor module that measures a voltage value and a current value. The converter control module may calculate a current instruction value by using the inputted power instruction value and the voltage value measured by the first sensor module, and generate the PWM signal for controlling the switch of the converter based on an error value between the current instruction value and the current value measured by the first sensor module.
Furthermore, the power conversion system may further include an inverter control module that outputs a PWM signal for controlling a switch of the inverter. After receiving a voltage instruction value, the inverter control module may generate a current instruction value based on the voltage instruction value and generate the PWM signal for controlling the inverter based on the generated current instruction value.
Further, the power conversion system may further include a second sensor module that measures a output voltage value of the converter and a third sensor module that measures a output current value of the inverter. The inverter control module may control an error value between the inputted voltage instruction value and the voltage value measured by the second sensor module to be zero and calculates a first current instruction value which makes the error value zero, obtain a feed-forward term by dividing the power instruction value received from the converter control module by a load voltage value and multiplies the feed-forward term by an estimation phase of a general electric power system, thereby obtaining a second current instruction value, and generate the PWM signal for controlling the switch of the inverter based on an error value between a third current instruction value obtained by adding the first current instruction value to the second current instruction value and the current value measured by the third sensor module.
Furthermore, the first sensor module may be installed between a fuel cell and the converter. The measured voltage value and current value may serve as a converter input voltage value and a converter input current value, respectively. The current instruction value may serve as a converter input current instruction value.
Further, the first sensor module may be installed between the converter and the inverter. The measured voltage value and current value may serve as a DC link voltage value and a DC link current value, respectively. The current instruction value may serve as a DC link current instruction value.
Furthermore, a voltage value measured by the second sensor module may serve as a DC link voltage value, and a voltage value measured by the third sensor module may serve as a inverter output current value.
Further, the external power supply is a fuel cell.
Furthermore, the converter may take any one of various forms such as a boost-type converter, a boost converter, a half-bridge converter, a full-bridge converter, and a push-pull converter so as to boost a low DC voltage to a DC voltage having a predetermined voltage level.
In accordance with another aspect of the present disclosure, there is provided a control method of a power conversion system (PCS) including a converter that converts a voltage of an input power supply, and an inverter that converts an output voltage of the converter into a AC voltage. The control method includes: receiving a power instruction value; and generating a PWM signal for controlling the converter based on the received power instruction value.
Further, the step of generating the PWM signal for controlling the converter may include: measuring a voltage value and a current value for controlling the converter; calculating a current instruction value by using the inputted power instruction value and the measured current value; and generating a PWM signal for controlling a switch of the converter based on an error value between the current instruction value and the measured current value.
Furthermore, the control method may further include receiving a voltage instruction value; and generating a PWM signal for controlling the inverter based on the voltage instruction value and the current instruction value which is calculated using the voltage instruction value.
Further, the control method may further include measuring a voltage value and a current value for controlling the inverter; calculating a first current instruction value for allowing an error value between the inputted voltage instruction value and the measured voltage value to be zero; obtaining a feed-forward term by dividing the inputted power instruction value by a load voltage value, and multiplying the feed-forward term by a estimation phase of a general electric power system, thereby obtaining a second current instruction value, and generating the PWM signal for controlling the switch of the inverter based on an error value between a third current instruction value obtained by adding the first current instruction value to the second current instruction value and the measured current value.
Further the measured voltage value and current value may serve as a converter input voltage value and a converter input current value, respectively, and the current instruction value may serve as a converter input current instruction value.
Furthermore, the measured voltage value and current value may serve as a DC link voltage value and a DC link current value, respectively, and the current instruction value may serve as a DC link current instruction value.
Further, the measured voltage value may serve as a DC link voltage value, and the measured current value may serve as an inverter output current value.
In accordance with still another aspect of the present disclosure, there is provided a program stored in a computer-readable storage medium that executes a control method of a power conversion system (PCS) including a converter for converting a voltage of an input power supply, and an inverter for converting an output voltage of the converter into an AC voltage. The program includes: receiving a power instruction value; and generating a PWM signal for controlling the converter based on the received power instruction value.
Further, the step of generating the PWM signal for controlling the converter may include: measuring a voltage value and a current value for controlling the converter; calculating a current instruction value by using the received power instruction value and the measured current value; and generating a PWM signal for controlling a switch of the converter based on an error value between the current instruction value and the measured current value.
Furthermore, the program may further include receiving a voltage instruction value; and generating a PWM signal for controlling the inverter based on the voltage instruction value and the power instruction value.
As described above, in accordance with the present disclosure, there is provided a new control method of eliminating a low frequency ripple current in a power conversion apparatus.
According to the present disclosure, if a control method of the power conversion apparatus is changed, it is possible to completely eliminate the low frequency ripple current without any additional hardware or cost.
Further, a converter controls a current and an inverter controls a DC link voltage and an inverter output current by means of an input power instruction instead of a conventional algorithm for controlling a current and a voltage in a converter.